There is presently considerable interest in the use of electrical domain techniques for compensation or equalisation of various sources of signal degradation in optical transmission systems, including chromatic dispersion, nonlinear propagation effects, and polarisation mode dispersion (PMD). In particular, the preservation of full amplitude and phase information in the conversion of electrical signals into corresponding optical signals at the transmitter, and in the corresponding optical-to-electrical conversion at the receiver, facilitates the use of a range of electrical signal processing techniques for equalisation and recovery of transmitted information signals. For example, the preservation of phase information enables the use of orthogonal frequency division multiplexing (OFDM) for coding and equalisation of information signals transmitted over optical channels.
Preservation of phase information is readily achieved through the use of coherent optical technologies. Coherent optical heterodyne receivers, for example, utilise a local optical oscillator, ie a suitable laser located at the receiver, having a frequency almost equal to that of the optical signal to be detected. The output field from the local laser is combined with the received signal field, and the combined signal directed to an optical-to-electrical converter, such as a photodiode. The process of so-called “square-law detection” occurring at the photodiode (ie conversion of optical intensity into electrical current or voltage) causes mixing between the local oscillator field and the received signal field, whereby the optical signal is converted into an equivalent electrical signal within the radio frequency (RF) domain. However, coherent optical heterodyne receivers are generally considered to have a number of practical disadvantages, particularly in terms of cost and complexity, which preclude their widespread deployment in optical communications systems. In particular, coherent receivers require a local oscillator (as previously noted), as well as polarisation stabilisation and frequency/phase locking of the optical local oscillator.
It is therefore desirable to avoid the need to employ coherent heterodyne receivers within optical communications systems. In particular, direct detection receivers, which require only a photodiode and associated electronics, are far simpler, less costly, more robust, and importantly are wavelength independent. In order to enable direct detection receivers to be used, it is desirable to transmit a suitable optical carrier along with the optical signal band. It is further desirable that the transmitted optical carrier and optical signal band are derived from a single optical source in order to avoid excessive phase noise in the received electrical signal which may result from finite laser linewidth. A further desirable characteristic of the transmitted optical signal is that it has a spectrum which is structured so as to avoid, or at least minimise, degradation of the received electrical signal which may result from mixing between signal components during the process of square-law detection. Additionally, it is desirable to maximise the utilisation of available bandwidth in the electrical domain, and of the associated signal processing capacity, in order to increase the available information capacity of the system reduce power consumption, reduce cost, minimise complexity, and so forth. A number of these desirable features result in conflicting design constraints, requiring trade-offs or compromises to be made in the implementation of suitable optical transmitters.
Accordingly, there is need for alternative and/or improved apparatus and methods for generating optical signals that are able to provide better overall performance in view of the various competing requirements and constraints.